There is considerable interest in phase shift masks as a route to extending the resolution, contrast and depth of focus of lithographic tools beyond what is achievable with the normal chrome mask technology. In the phase shift mask the quartz substrate plate is etched or coated so as to allow interference effects to be employed. The etch or coating has a thickness so that an optical path difference may be established between adjoining regions of just 1/2 wavelength, resulting in destructive interference at the boundary of the two regions. There has been a great deal of work to quantify and exploit these effects for example in alternating phase, rims and other structures. A problem that has so far hindered the introduction of phase shift masks into full scale microcircuit designs has been the difficulty of generating design tools to facilitate mask layout automatically for the circuit designer. For this reason there is a focus on the use of so called attenuating phase shift masks.
In the attenuating phase shift mask, the chrome layer is replaced by a slightly transparent layer in combination with a 1/2 wavelength phase shift. This mask has the properties that there is only one patterning step as for the chrome mask and that every edge has enhanced contrast due to the 180 degree phase shift effect. At present such masks are typically prepared by a combination of a slightly transparent, for example very thin chrome layer, coupled with a quartz etch to produced the desired phase shift. This method requires a high degree of control of both layer and etch process.